1. Field of the Invention
The present invention generally relates to a sensor assembly for detecting the angle of rotation of elements relative to each other and, more particularly, to the rotational angle sensor assembly for use in construction machinery and industrial machinery for detecting the angle of rotation of a rotating element such as, for example, a rotary shaft or an oscillating shaft while the latter is operated under a loaded condition.
2. Description of the Prior Art
As an instrument for measuring the angle of rotation of a rotary shaft or an oscillating shaft, a rotational angle detecting potentiometer has long been used. The rotational angle detecting potentiometer includes an electroconductive brush movable together with, for example, a rotary shaft and an electric resistor held in sliding contact with the brush. The potentiometers provides a voltage signal represented by the resistance of the resistor which varies depending on the position of the electroconductive brush, so that the angle of rotation of the rotary shaft can be determined in reference to the voltage signal.
The standard rotational angle detecting potentiometer hitherto used makes use of an oil retaining metal or a relatively small rolling bearing in order to permit the rotary shaft to be rotatably supported relative to a housing and is incapable of allowing the rotary shaft to be loaded with a relatively high load. When a relatively high load is imposed on the rotary shaft in one or both of axial and radial directions thereof, a relatively large error tends to occur in the detected angle of rotation.
In order to alleviate the foregoing problem, the rotational angle sensor assembly of a structure shown in FIG. 4 has been suggested by the applicant. Referring to this figure, the rotational angle sensor assembly shown therein includes a rotary shaft 52 having a large diametric portion 52a formed therein so as to protrude radially outwardly therefrom and supported by a housing 51 through bearings 53. The housing 51 is in the form of a cap sufficient to accommodate therein both the large diametric portion 52a of the rotary shaft 52 and the bearing 53 and having an opening closed by a lid 54. The lid 54 is secured to the housing 51 by means of a plurality of set bolts 58. First and second rotational angle detecting members 56 and 57 altogether forming a rotational angle detecting means 55 are mounted on an inner surface of the lid 54 and an end face of the rotary shaft 52, respectively. One of the detecting members 56 and 57 is made up of an electric resistor and the other is made up of an electroconductive brush. The bearings 53 are positioned within the housing 51 coaxially one above the other with an annular space left therebetween. Within the annular space between the bearings 53, a flange 52b radially outwardly extending from the rotary shaft 52 is interposed between respective inner races of those bearings 53 whereas an annular spacer 59 is interposed between respective outer races of those bearings 53.
According to the structure shown in FIG. 4, since the bearings 53 are preloaded at a predetermined position and, also, since the rotary shaft 52 is formed with the large diametric portion 52a to enable the bearings 53 of a relatively large size to be employed, the rigidity with which the rotary shaft 52 is supported can increase advantageously and, even in a condition in which a load acts on the rotary shaft 52, the angle of rotation of the rotary shaft 52 relative to the housing 51 can advantageously be detected accurately and with no error occurring.
However, the rotational angle sensor assembly of the structure shown in and described with reference to FIG. 4 has the following disadvantage. Specifically, since the housing 51 and the lid 54 carrying the first rotational angle detecting member 56 including a potentiometer substrate and others are members separate from each other, any possible error in mounting the lid 54 to the housing 51 such as resulting from, for example, a play occurring between one or more set bolts 58 and associated bolt holes may brings about a detrimental displacement of the reference or base position from which an output representative of the angle of rotation of the rotary shaft 52 relative to the housing 51 is based. In other words, where the rotational angle detecting means 55 is assembled in the form of a potentiometer made up of an electric resistor and an electroconductive brush, an electrically neutral position (normally, a zero reading position of the potentiometer) relative to the housing 51 may detrimentally displace as a result of an error in mounting of the lid 54 relative to the housing 51. In addition, the employment of the lid 54 brings about the necessity to use, other than the lid 54, the set bolts 58 and an O-ring seal 60 to be interposed between the lid 54 and the housing 51, resulting in increase of the number of component parts.
Also, in order for the bearings 53 to be set in position under a pre-loaded condition, the annular spacer 59 and/or the flange 52b between the two bearings 53 must have a thickness carefully designed to be consistent with the size of the bearings and do thus require a precise machining accuracy, resulting in increase of the cost of manufacture.
In view of the foregoing, the present invention is intended to provide an improved rotational angle sensor assembly effective to minimize variation of the reference or base position from which the output representative of the angle of rotation of, for example, the rotary shaft relative to the housing, to reduce the number of component parts employed and also to effectively and sufficiently support the load acting on the rotary shaft.
Another important object of the present invention is to provide an improved rotational angle sensor assembly of the type referred to above, which does not substantially require a precise machining accuracy in mounting the bearings under a pre-loaded condition and which can be easily and inexpensively assembled using a simplified machining.
In order to accomplish these objects of the present invention, there is provided a rotational angle sensor assembly which includes a generally cup-shaped housing having an opening leading into a hollow defined therein, a rotary shaft having first and second ends opposite to each other, and a rotational angle detecting means including first and second rotational angle detectors and operable to detect the angle of rotation between the first and second rotational angle detectors. The first rotational angle detector is disposed at a bottom of the hollow of the housing. The second rotational angle detector being mounted on the first end of the rotary shaft within the hollow of the housing. A first portion of the rotary shaft adjacent the second end thereof extends outwardly from the opening of the housing. A bearing means is accommodated within the hollow of the housing under a pre-loaded condition for rotatably supporting the rotary shaft.
According to the present invention, the angle of rotation of the rotary shaft relative to the housing is detected in terms of the angle of rotation between the first and second rotary angle detectors provided in and on the housing and the rotary shaft, respectively. Since the housing is of a generally cup-shaped structure with the first rotational angle detector disposed directly at the bottom of the hollow of the housing, unlike the conventional design in which a rotational angle detector is mounted on a lid which is in turn secured to a housing, the present invention is advantageous in that no variation in output indicative of the angle of rotation due to an error in mounting will occur which would otherwise result from a mounting error. Also, the housing is of a solid one-piece structure, not only is the need to use any lid eliminated, but the need to use any bolts otherwise necessary to secure the lid to the conventional housing and any sealing member otherwise necessary to be interposed between the lid and the conventional housing are also eliminated, thereby reducing the number of component parts advantageously.
Also, since the bearing means is pre-loaded as hereinbefore described, the rotary shaft is substantially free from any rattling motion during rotation thereof and the angle of rotation can be accurately detected even under a loaded operating condition.
In a preferred embodiment of the present invention, a spring member may be employed for applying a pre-load to the bearing.
The use of the spring members is effective to apply a proper pre-load to the bearing means so that the latter can be pre-loaded at a predetermined pressure, without requiring any stringent dimensional accuracy such as required where a bearing is pre-loaded at a predefined position. For this reason, the need to highly accurately machine the housing, a shaft flange and/or a spacer is advantageously eliminated to thereby allow the rotational angle sensor assembly to be manufactured inexpensively with a simplified machining procedure.
In the practice of the present invention, the rotary shaft may have a portion stepped to provide a large diametric portion at one end thereof where the bearing means supports the rotary shaft rotatably, having a diameter greater than that of the remaining portion of the rotary shaft.
Where that portion of the rotary shaft that is supported by the bearing means is represented by the large diametric portion of the rotary shaft, the bearing means of a relatively large size can be employed. The use of the bearing means of a relatively large size is effective in that even though a load acting on the rotary shaft is relatively high, the bearing means can support such high load and therefore, an accurate detection of the angle of rotation is possible even when the rotary shaft is loaded by the high load.
Preferably, the bearing means includes first and second bearings having inner and outer races with a plurality of rolling elements interposed therebetween. These first and second bearings are positioned within the hollow of the housing one above the other in a direction parallel to the rotary shaft, with the first bearing positioned adjacent the opening of the hollow of the housing. In such case, a stop ring is used to secure the outer race of the first bearing to the housing. In this design, the spring member for applying a predetermined pre-load to the bearing means is interposed between a bottom face of the hollow of the housing and the outer race of the second bearing adjacent the bottom face of the hollow of the housing or, alternatively, between the stop ring and the outer race of the first bearing adjacent the stop ring. Also, a sealing member may be used to fill up an annular space between an inner peripheral surface of the housing adjacent the opening and an outer peripheral surface of the rotary shaft.
The use of the two bearings in combination with the spring member is effective to accomplish the pre-loading with a predetermined pressure.
Also in a preferred embodiment of the present invention, one of the first and second rotational angle detectors includes an electric resistor and the other of the first and second rotational angle detectors includes a slide brush, so that the rotational angle detecting means can serve as a potentiometer.
Where the rotational angle detecting means is in the form of a potentiometer, the base position from which an output representative of the angle of rotation of the rotary shaft relative to the housing is based may be considered an electrically neutral position. With respect to this electrically neutral position relative to the housing, the rotational angle sensor assembly such as that the design in which the rotational angle detector is provided in the generally cup-shaped solid one-piece housing is substantially free from dimensional variation among the component parts thereof as compared with the rotational angle detector is provided on the lid that is separate from the housing. Although the rotational angle detecting means in the form of a potentiometer is simple in structure, any possible change in a biasing force used to urge the slide brush so as to keep the slide brush in sliding contact with the resistor and/or the electroconductive element would eventually result in an error in measurement of the angle of rotation of the rotary shaft. However, according to the present invention, since the bearing means is pre-loaded, the biasing force applied to the slide brush can be stabilized even though a load acting on the rotary shaft is relatively high, allowing a precise detection of the angle of rotation to be achieved advantageously.